This application claims priority over provisional patent application Serial No. 60/178,281 filed Jan. 27, 2000.
The purpose of this invention is to provide a means of transmitting instructions to downhole tools by means of drill string rotation encrypted commands. Mud-Pulse Measure-while-drilling (MWD) systems typically require a means of communicating to the tool during drilling operations to reconfigure the tool""s operation. This is traditionally accomplished by transmitting an encoded message via cycling the mud pumps on and off at prescribed intervals.
In the past it has been common to instruct downhole tools to change modes of operation or perform or modify different functions by means of varying the flow of fluids being pumped down the drill string. Pressure switches or transducers that measure a differential pressure across the tool when fluids are flowing are used to sense this flow. The flow is stopped and started to send desired commands. Generally, such no-flow and flow states can be interpreted as the equivalent of a xe2x80x9c0xe2x80x9d or a xe2x80x9c1xe2x80x9d in a binary or binary-like code. Likewise, accelerometers that measure vibration can at times be used in place of pressure transducers because there are low level vibrations induced in a drill string and tools mounted in it when fluid flows.
This invention provides a method and apparatus for encrypting and receiving coded messages to downhole tools by measuring modulation of a downhole condition induced as by rotating the rotary table or turntable carrying the drill string at the surface of the earth which in turn rotates the drill string. This rotation is transmitted by the drill string to the downhole end of drill string and such rotation induces modulation of one or more downhole conditions that may be measured. Such downhole conditions may, for example, be linear or angular vibration levels, angular rate around the drill axis, directional tool face (relative direction of tool with respect to a true or magnetic North reference) or high-side tool face (relative rotation about the drill string with respect to gravity. This method has many advantages over the mud pump controlled (fluid flow controlled) messages as the rotary drive mechanisms can be more easily and more precisely controlled.
For instance, it is not uncommon to encrypt fluid flow messages with minutes of flow and no flow times where flow and no flow times might represent coded bits of a message. Measuring linear vibration induced from fluid flow is also now used to send messages to down hole tools, but this technique seriously loses sensitivity with large drill strings. Such methods still depend on modulation of the mud flow rate by starting and stopping the mud pumps. Measuring linear and/or angular vibration induced by rotating the drill string is far less sensitive to drill string size.
Downhole magnetic direction sensors are sometimes used to detect drill string rotation or the absence of drill string rotation and such information is used to command simple on-off functions for downhole tools. Such schemes detect that rotation is or is not occurring. Such schemes require non-magnetic drill string elements and have other complications as well
Rotary tables can be easily controlled for 15-second periods of rotation-on and rotation-off. Thus, very expensive drill rig time can be saved. In addition, more complex encrypting concepts to even further shorten messages become possible because of the added precision possible with rotary drill string drive mechanisms (as opposed to the sluggish nature of controlling the large amounts of fluid needed to get adequate detection down hole).
One embodiment of this invention is based on the use of angular or linear vibration sensors to measure downhole vibration conditions and to use the resulting signals to decode messages transmitted to downhole tools by means of drill string rotation on-off-on at different levels for encrypting such messages. In other embodiments, an inertial angular rate sensor, typically a gyroscope, is used to sense commanded rotation angular rates of the drill string.
Accordingly, it is one major object of the invention to provide a method for downward communication in a borehole, comprising the steps:
a) imparting a series of rotary motions to an upper portion of the string, such rotary motions representing at least two levels of a coded data sequence, the rotary motions imparted to the string upper portion effecting generally comparable motions at or proximate the lower end of the drill string, or at a string lower portion,
b) the rotary motions at or proximate the lower end of the drill string, or string lower portion, effecting a downhole detectable condition or conditions indicative of such imparted rotary motions,
c) detecting said condition or conditions to determine a corresponding coded data sequence,
d) and processing said corresponding data sequence to recover the imparted coded data sequence, from which a unique transmitted message is determinable.
More generally, the method for transmitting a message or information between upper and lower zones in a borehole includes the steps:
a) effecting rotary displacement of the pipe string at said upper zone in a manner to effect a corresponding rotary pipe displacement at said lower zone,
b) said displacement representing at least two levels of a coded data sequence containing said message.
The method typically also includes providing an accelerometer detecting vibrational acceleration resulting from pipe string rotation, and having an output, there being sampling means responsive to the accelerometer output to sample at time intervals in excess of 50 times per second, there also being a filter to filter and average the output of the sampling means, and including the step of determining from the input of the filter whether pipe string rotation is occurring, and if such rotation is determined as occurring, then monitoring the output of the accelerometer to detect transitions above and below a threshold, for message determination.
Further objects include filtering and amplifying the downhole accelerometer output; repeatedly sampling that digitized output to produce a further output, and then subjecting that further output to progressive averaging to produce a progressively averaged output in the form of pulses; monitoring that progressively averaged output to determine whether it is continuously above a selected threshold for a predetermined time period, in which event, prospective message pulses are determined as being transmitted; and subjecting the determined prospective message pulses to pulse edge and pulse width discrimination, as a further determination of message validity.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: